Polyolefins are useful in any number of everyday articles. However, one drawback to many polyolefins, especially propylene homopolymers and some propylene copolymers, is their relatively high glass transition temperature. This characteristic makes these polyolefins brittle, especially at low temperatures. Many applications of polyolefins benefit from having useful properties over a broad range of temperatures; consequently, there is a need to provide polyolefins that can maintain desirable characteristics such as high or low temperature performance, etc., while maintaining or improving upon the impact strength and toughness at lower temperatures. In particular, it would be advantageous to provide a propylene polymer possessing improved toughness and or high use temperature without sacrificing its other desirable properties.
Addition of a plasticizer or other substance to a polyolefin is one way to improve such properties as impact strength and toughness. Some patent disclosures directed to such an end are U.S. Pat. No. 4,960,820; U.S. Pat. No. 4,132,698; U.S. Pat. No. 3,201,364; WO 02/31044; WO 01/18109 A1; and EP 0 300 689 A2. These disclosures are directed to polyolefins and elastomers blended with functionalized plasticizers. The functionalized plasticizers are materials such as mineral oils which contain aromatic groups, and high (greater than −20° C.) pour point compounds. Use of these compounds typically does not preserve the transparency of the polyolefin, and impact strength is often not improved.
WO 98/44041 discloses plastic based sheet like material for a structure, especially a floor covering, which contains in a blend a plastic matrix comprising a chlorine free polyolefin or mixture of polyolefins and a plasticizer characterized in that the plasticizer is an oligomeric polyalphaolefin type substance.
Other background references include EP 0 448 259 A, EP 1 028 145 A, U.S. Pat. Nos. 4,073,782, and 3,415,925.
What is needed is a polyolefin with lower flexural modulus, lower glass transition temperature, and higher impact strength near and below 0° C., while not materially influencing the peak melting temperature of the polyolefin, the polyolefin crystallization rate, or its clarity, and with minimal migration of plasticizer to the surface of fabricated articles. A plasticized polyolefin according to this invention can fulfill these needs. More specifically, there is a need for a plasticized polypropylene that can be used in such applications as food containers, medical devices, durable household goods, and toys.
Likewise, a plasticized polyolefin with improved softness, better flexibility (especially lower flexural modulus), a depressed glass transition temperature, and/or improved impact strength (especially improved Gardner impact), where the melting temperature of the polyolefin, the polyolefin crystallization rate, or its optical properties (especially clarity and color) are not influenced and with minimal migration of the plasticizer to the surface of articles made therefrom is desirable.
It would be particularly desirable to plasticize polyolefins by using a simple, non-reactive compound such as a paraffin. However, it has been taught that aliphatic or paraffinic compounds would impair the properties of polyolefins, and was thus not recommended. (See, e.g., CHEMICAL ADDITIVES FOR PLASTICS INDUSTRY 107-116 (Radian Corp., Noyes Data Corporation, NJ 1987); WO 01/18109 A1).
Mineral oils, which have been used as extenders, softeners, and the like in various applications, consist of thousands of different compounds, many of which are undesirable in a lubricating system. Under moderate to high temperatures these compounds can volatilize and oxidize, even with the addition of oxidation inhibitors.
Certain mineral oils, distinguished by their viscosity indices and the amount of saturates and sulfur they contain, have been classified as Hydrocarbon Basestock Group I, II or III by the American Petroleum Institute (API). Group I basestocks are solvent refined mineral oils. They contain the most unsaturates and sulfur and have the lowest viscosity indices. They define the bottom tier of lubricant performance. Group I basestocks are the least expensive to produce, and they currently account for abut 75 percent of all basestocks. These comprise the bulk of the “conventional” basestocks. Groups II and III are the High Viscosity Index and Very High Viscosity Index basestocks. They are hydroprocessed mineral oils. The Group III oils contain less unsaturates and sulfur than the Group I oils and have higher viscosity indices than the Group II oils do. Additional basestocks, named Groups IV and V, are also used in the basestock industry. Rudnick and Shubkin (Synthetic Lubricants and High-Performance Functional Fluids, Second edition, Rudnick, Shubkin, eds., Marcel Dekker, Inc. New York, 1999) describe the five basestock Groups as typically being:    Group I—mineral oils refined using solvent extraction of aromatics, solvent dewaxing, hydrofining to reduce sulfur content to produce mineral oils with sulfur levels greater than 0.03 weight %, saturates levels of 60 to 80% and a viscosity index of about 90;    Group II—mildly hydrocracked mineral oils with conventional solvent extraction of aromatics, solvent dewaxing, and more severe hydrofining to reduce sulfur levels to less than or equal to 0.03 weight % as well as removing double bonds from some of the olefinic and aromatic compounds, saturate levels are greater than 95-98% and VI is about 80-120;    Group III—severely hydrotreated mineral oils with saturates levels of some oils virtually 100%, sulfur contents are less than or equal to 0.03 weight % (preferably between 0.001 and 0.01%) and VI is in excess of 120;    Group IV—poly(alpha-olefin) hydrocarbons manufactured by the catalytic oligomerization of linear olefins having 6 or more carbon atoms. In industry however, the Group IV basestocks are referred to as “polyalphaolefins” are generally thought of as a class of synthetic basestock fluids produced by oligomerizing C4 and greater alphaolefins;and    Group V—esters, polyethers, polyalkylene glycols, and includes all other basestocks not included in Groups I, II, III and IV.
Other references of interest include: U.S. Pat. No. 5,869,555, U.S. Pat. No. 4,210,570, U.S. Pat. No. 4,110,185, GB 1,329,915, U.S. Pat. No. 3,201,364, U.S. Pat. No. 4,774,277, JP01282280, FR2094870, JP69029554, Rubber Technology Handbook, Werner Hoffman, Hanser Publishers, New York, 1989, pg294-305, Additives for Plastics, J. Stepek, H. Daoust, Springer Verlag, New York, 1983, pg-6-69.
U.S. Pat. No. 4,536,537 discloses blends of LLDPE (UC 7047), polypropylene (5520) and Synfluid 2CS, 4CS, or 6CS having a viscosity of 4.0 to 6.5 cSt at 100° F./38° C., however the Synfluid 4CS and 8CS are reported to “not work” (col 3, ln 12).
In another aspect is also desirable to have flexible polyolefins (typically polypropylene) that can withstand sterilizing amounts of radiation. Typical polypropylene tends to soften and deform when sterilized at high temperature by steam or turn yellow and/or become brittle when treated with high energy radiation, particularly beta and gamma radiation.
Beta radiation, such as from an electron beam, or gamma radiation, such as from a cobalt-60 source, is often used to sterilize medical equipment. This is a particularly convenient means of sterilization since the items may be packed in bulk, or in individually sealed clean packages, and irradiated after packaging. Such treatments yield sterile instruments and devices without the need for special handling or repackaging after sterilization. Thus, sterility and enhanced patient safety are assured. However, because polypropylene tends to degrade when exposed to sterilizing levels of radiation, such treatment is generally inappropriate for medical devices incorporating polypropylene components or for medical devices packaged in polypropylene containers.
But for this limitation, polypropylene would be very useful for making a tremendous number of useful items including syringe barrels, culture dishes, tissue culture bottles, intravenous catheters and tubing, and bags or bottles, surgical probes, suture material, and other goods.
The potential usefulness of polypropylene has been recognized for some time. Others in the field have attempted to overcome the property limitations by numerous means. In U.S. Pat. No. 4,110,185, for example, Williams, Dunn, and Stannett describe the use of a non-crystalline mobilizing agent in polypropylene formulations to increase the free volume of the polymer and prevent radiation embrittlement (see also U.S. Pat. No. 4,274,932 and U.S. Pat. No. 4,467,065). In U.S. Pat. No. 4,845,137, Williams and Titus describe a polypropylene composition which is stable to sterilizing radiation, comprising polypropylene of narrow molecular weight distribution (Mw/Mn), a liquid mobilizing additive, a hindered amine compound, and a clarifying agent. While these additives generally appear to enhance radiation-tolerance, mobilizing additives tend to be oily or greasy. This can contribute to processing difficulties and product flaws.
Other inventions attempting to stabilize polypropylene against the effects of high energy radiation employ syndiotactic polypropylene. EP-A2-0 431 475, describes making a radiation resistant polypropylene resin composition suitable for the preparation of molded articles in which physical properties “scarcely deteriorate during sterilization by radiation” by utilizing substantially syndiotactic polypropylene. The composition may also include a phosphorous containing anti-oxidant, an amine containing antioxidant, and a nucleating agent.
JP 04-214709 apparently describes ethylene/propylene copolymers with at least 50% syndiotacticity which have improved radiation tolerance. Such copolymers are produced by specific chiral metallocene-type catalysis and are preferably compounded with phosphorous or amine-containing antioxidants for best radiation tolerance.
U.S. Pat. No. 5,340,848 describes a radiation resistant polypropylene resin composition comprising a polypropylene having a substantially syndiotactic structure with optional anti-oxidants and/or nucleating agents.
WO 92/14784 describes blends of from 30 to 40 weight percent of an ethylene-based copolymer with 70 to 30 weight percent of a propylene-based copolymer for use in heat seal applications.
These references indicate that a simple, cost effective system to provide radiation tolerant polypropylene has long been sought. Ideally, such a polypropylene composition would provide products that are clear and would be dimensionally stable at elevated temperatures. Such products could optionally be subjected to sterilization by means other than radiation without softening or deformation or significant deterioration of optical and or strength properties. It would further benefit the makers of polypropylene articles if the polymer blend used for forming would not tend to foul the molding equipment with oil or grease. Users of the final formed products, as well as makers of such articles, would benefit if such polymer compounds would not exude oil or grease from the surface of molded parts, films, or packaging. Such articles would be particularly attractive to the medical and food packaging industries.